Why a marimo? It really doesn't do anything for the water, nor the algae.. silly fad that was brought to America lol. :)

Okay, I missed the part you mentioned turning off the filter. Just keep an eye on him, if he isn't getting any better in a couple of days I would place him back into the small tank with some decoration or two and see how he does there. If he starts to get active and swims around, then you know he prefers that.. in which case you can get another betta for the larger tank and see how the new guy likes it :)

It is algae, doesn't filter ammonia nearly enough to register, and as for out competing the algae, honestly it doesn't. Not a single ball in a tank. In lakes with thousands of these it works the way you think.. but in a small tank at home it really is just a decoration. It's a gimmick, they sell them as "pets" in other countries, people have necklaces with a marimo in it. It's fine and fun to have, to have it.. but if having algae problems or water quality issues, it's best to go for actual aquatic plants and snails.

It has been registered plenty of times to do a good job outcompeting algae. Also shrimp love Marimo. Also when I removed my Marimo ball Algae cam back. Forgot to add sorry it turns out you and me are the only one with that sig and I got confused.

Thank you for all the advice, unfortunately on Sunday I came home and ran to check on him and he was gone. I was heartbroken...poor little guy. The amonia levels and all other levels were great, the temp etc was fine. I think the filter was too strong and he wasn't big enough. That is what broke my heart, that he died because the filter was too strong and it was my fault. It has now been baffled (took a bit to figure that out :) ) I still have a Golden Mystery Snail in my tank and I am condsidering a new fish. I did a 50% water change before he died, should I do a full one before getting a new fish?
Thanks.